What is this absolute reticence for using the tool of the trade? Yes, you may be able to sterilize a finely granulated substrate using boiling water or steam. But you will have more and more trouble as your elevation increases.
Many microorganisms produce endoscope. The natural purpose of endoscope is to resist environmental extremes.
Boiling water may kill 99.99 percent of all endoscope after an hour or so.
But it only takes a single survivor and there may be billions of spores in your substrate. Wet heat, and wet heat under pressure not only speeds up the process but invades the micro crevices in the substrate.
I can't believe I'm saying this, but yes, if you take care to prompt the endospores into germination first, by ensuring the substrate is moist and kept in the low eighties for 12 to 24 hours, you may be able to kill all contaminants in a grain medium by boiling after the spores germinate.
Several hours boiling may do it. If you insist. Set up your substrate, wait for a day but not so long as to let your substrate sour thus changing the pH, then boil, then let it cool and boil yet again, you will most likely arrive at a sterile substrate.
Or, you can get a pressure cooker and make your life, far easier.
Those pressure cookers will also cook artichokes to perfection.